Answer
The shortest distance between two points; It allows us to study the curvature of space.
Work Step by Step
A geodesic is defined as the shortest distance between two points. Examples in the book include geodesics on on a flat plane (straight lines) and on the surface of a sphere (arcs of great circles).
General relativity tells us that space-time is curved. If we measure geodesics (e.g., study how objects or light rays behave near large masses) we gather information about how the mass affects the curvature of local space-time.
